package dujiu.luogu.sequence;

import java.util.Scanner;

public class P5708 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double a = scanner.nextDouble();
        double b = scanner.nextDouble();
        double c = scanner.nextDouble();

        double p =  (a + b + c) /2;
        double res = Math.sqrt((p) * (p - a) * (p - b) * (p - c));
        System.out.printf("%.1f",res);
    }
}
